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388 ALASKAN GLACIER STUDIES
still engaged in the same work. These deposits may be spoken of as glacial deposits because they were formerly and are still being supplied almost entirely by streams from melting glaciers farther east, notably the Scott, Sheridan, and Martin River Glaciers, and smaller ice tongues along the mountain front, and the Miles, Childs, Goodwin, Allen, Heney, and other glaciers of the Copper River valley. The Copper River, a great overburdened glacial stream, has built an enormous delta, coalescing with the deltas of Eyak and Martin Rivers. Much of the finer material is carried out beyond this delta and is drifted westward along shore and, since Orca Inlet is the first great opening in the coast, a large amount of the sediment has been driven into it, so nearly filling it that only narrow, shallow channels are occupied by water at low tide.
The third deposit of glacial origin below sea level, is Middle Ground Shoal, a triangular area projecting into Orca Bay from Hawkins Island Cutoff, and interpreted as a tidal delta. It is composed of fine sand and clay similar to that filling Orca Inlet and derived from the same source. This sediment is part of the material that was drifted into Orca Inlet in suspension, and swept through Hawkins Island Cutoff, coming to rest where the tidal currents are checked in the broad outer portion of Orca Bay.
This deltoid shoal, 5 miles wide near the land, extends northward 3 J miles into Orca Bay. It is covered at mean low tide by water only l£ to 15 feet deep, and has narrow distributary channels 21 to 30 feet in depth. At the front and edges of this tidal delta the depth increases rapidly, reaching 156 feet in a little over a quarter of a mile and 300 or 400 feet a little further out. The triangular deposit has, therefore, been built in deep water and is still being extended. Its margins slope 7£° to 10°. The channel in Hawkins Island Cutoff and the distributary channels on the tidal delta are kept open by the tide, which moves both ways through the strait.